1. Field of the Invention
The present invention relates to a light emitting diode made of an AlGaInP-based compound semiconductor, and particularly to a light emitting diode with high luminance which can prevent exfoliation of a transparent conductive film made of a metal oxide, and is manufactured inexpensively.
2. Description of the Related Art
Most of conventional light emitting diodes are green GaP (gallium phosphide) and red AlGaAs (aluminum gallium arsenide). In recent years, however, since a technology wherein GaN (gallium nitride)-based or AlGaInP (aluminum gallium indium phosphide)-based crystal layers are grown by means of MOVPE (Metal Organic Vapor Phase Epitaxy) technique is developed, an LED having a desired light emission wavelength belongs to orange, yellow, green, blue and the like other than red can be manufactured.
To obtain high luminance in LED, it is important to emit homogeneously light in a chip. For this purpose, it is required to obtain good current spreading. For achieving the purpose, a manner for increasing a thickness of a current spreading layer (being called by another name of window layer) is known. However, a cost for forming a current spreading layer becomes expensive in view of manufacturing epitaxial wafer for LED use. After all, there is a disadvantage of increasing a cost for manufacturing epitaxial wafer for LED use.
To decrease a manufacturing cost of LED, it is desirable to reduce a thickness of current spreading layer. For this purpose, an epitaxial layer having low resistance is necessary, so that an epitaxial layer having a high carrier concentration is required. In AlGaInP and GaN, however, it is difficult to grow a p-type epitaxial layer having a high carrier concentration. On one hand, another type of semiconductor may be used for fabricating LED so far as the questioned semiconductor has the above-described necessary characteristic properties. Unfortunately, any semiconductor satisfying such required characteristics has not yet been found.
Moreover, it is also known in GaN-based LEDs that a metallic film is used for a current spreading layer. In this case, however, it is required that a thickness of the metallic film is made to be very thin to increase transmittance of light, so that current spreading effect decreases. On the other hand, when enhancement of current spreading effect is intended, a thickness of such metallic film increases inevitably, so that light transmittive property is obstructed, resulting in a restriction of thickness. In addition, a metallic film is usually formed by vacuum evaporation technique. In this case, a prolonged time for evacuation becomes also a problem.
There is an ITO (Indium Tin Oxide) film being a metal oxide film having sufficient light transmittive characteristics and electrical characteristics for obtaining current spreading effect. Furthermore, there is an LED wherein the ITO film is used for a current spreading layer. According to the LED, since no epitaxial layer may be required for a current spreading layer, an LED having high luminance can be inexpensively produced.
(1) An LED Described in Japanese Patent Application Laid-Open No. 2002-344017.
FIG. 1 is a sectional view showing a structure of the LED described in the above Japanese patent application laid-open No. 2002-344017.
This LED 20 is a red LED having an emission wavelength of around 630 nm and which is prepared by lamination of an n-type GaAs substrate 1, an n-type (selenium (Se) doped) (Al0.7Ga0.3)0.5In0.5P cladding layer 2, an undoped (Al0.15Ga0.85)0.5In0.5P active layer 3, a p-type (Zn doped) (Al0.7Ga0.3)0.5In0.5P cladding layer 4 having 5×1017 cm−3 Zn concentration, a p-type GaP layer 5, a p-type InP layer 6, a transparent conductive film 7, an n-type electrode 8 formed on the whole surface of the bottom of a chip, and a p-type circular electrode 9 having a 150 μm diameter and formed on the top of the LED.
The respective layers extending from the n-type GaAs substrate 1 to the p-type AlGaInP cladding layer 4 are formed by means of MOVPE technique. A growth condition in the MOVPE technique is such that a growth temperature is 700° C., a growth pressure is 50 Torr, a growth rate in the respective layers ranges from 0.3 to 1.0 nm/s, and a V/III ratio ranges from 300 to 600, respectively.
The p-type GaP layer 5 is formed at 1×1018 cm−3 Zn concentration, 100 V/III ratio, 1 nm/s growth rate and in 2 μm thickness.
The p-type InP layer 6 is formed at 1×1018 cm−3 Zn concentration and which is provided as an underlying layer of the transparent conductive film 7, functioning to prevent exfoliation of the transparent conductive film 7 from an epitaxial wafer in case of dicing and the like.
The transparent conductive film 7 is made of an ITO film, and which is formed by vacuum evaporation technique. An evaporation condition for the ITO film is such that a substrate temperature is 250° C., an oxygen partial pressure is 4×10−4 Torr, and a thickness of about 200 nm.
The n-type electrode 8 is formed by evaporating gold-germanium with 60 nm thickness, nickel with 10 nm thickness, and gold with 500 nm thickness, respectively, in this order.
The p-type electrode 9 is formed by evaporating gold-zinc with 60 nm thickness, nickel with 10 nm thickness, and gold with 1000 nm thickness, respectively, in this order.
The LED 20 is fabricated by cutting out an epitaxial wafer with electrodes used for the LED formed in the above-described structure into 300 μm square chip size according to dicing. In a process for dicing and the like, the transparent conductive film 7 is cut out while maintaining adherence to the p-type InP layer 6. The LED 20 is die-bonded on the TO-18 stem, and the LED 20 is electrically connected to the TO-18 stem by wire-bonding.
According to the above-described conventional LED, however, a junction of the LED having pn junction and the transparent conductive film 7 becomes substantially npn, resulting in an appearance of series resistance due to barrier in the interface of the transparent conductive film 7 and the p-type InP layer 6. In this respect, since the Zn concentration (1×1018 cm−3) in the above-described transparent conductive film 7 is insufficient for an amount of reducing series resistance, a high operative voltage is required in light emission. Besides, a thickness (30 nm) for acquiring sufficient current spreading characteristics is required, so that there is a problem of appearing hindrance for attaining high luminance.
(2) Another Conventional LED
On one hand, to solve the above-mentioned problem, there is described a manner for driving an LED based on tunneling current by increasing extremely a carrier concentration of a semiconductor layer located on the uppermost of the LED (see ELECTRONICS LETTERS, 7 Dec. 1995 (pages 2210 to 2212).
Furthermore, there is described a method for fabricating an LED having high luminance, a low operative voltage, and high reliability in such a manner that a GaAs layer to which carbon (C) is added is used as the uppermost semiconductor layer, and carbon tetrabromide (CBr4) is used as a raw material for adding C (see Japanese patent application laid-open No. 1999-307810).
FIG. 2 is a sectional view showing a conventional LED wherein an ITO film is used.
This LED is a red LED having an emission wavelength of around 630 nm and which is prepared by formation of an n-type GaAs substrate 11, an n-type (Se doped) GaAs buffer layer (400 nm thickness, and 1×1018 cm−3 carrier concentration) 12, an n-type (Se doped) (Al0.7Ga0.3)0.5In0.5P cladding layer (300 nm thickness, and 1×1018 cm−3 carrier concentration) 13, an undoped (Al0.10Ga0.90)0.5In0.5P active layer (600 nm thickness) 14, a p-type (Zn doped) (Al0.7Ga0.3)0.5In0.5P cladding layer (300 nm thickness, and 5×1017 cm−3 carrier concentration) 15, a p-type (C doped) GaAs layer (25 nm thickness) 113, an ITO film 17 of a transparent conductive film, a circular p-type electrode 18, and an n-type electrode 19.
The respective layers (except for the ITO film 17, the p-type electrode 18, and the n-type electrode 19) are formed by means of MOVPE technique. A growth condition in the MOVPE technique is such that a growth temperature is 700° C., a growth pressure is 50 Torr, a growth rate in the respective layers ranges from 0.3 to 1.0 nm/s, and a V/III ratio ranges from 300 to 600, respectively.
Raw materials used in the growth according to MOVPE technique include organic metals such as trimethyl gallium (TMG) or triethyl gallium (TEG), trimethyl aluminum (TMA), and trimethyl indium (TMI); and hydride gases such as arsine (AsH3) and phosphine (PH3). Moreover, hydrogen selenide (H2Se) is used as a raw material for adding an additive (additive raw material) to an n-type layer such as the n-type GaAs buffer layer 12.
The ITO film 17 is a metal oxide film to be a current spreading layer, and which is formed in about 230 nm thickness at 300° C. film formation temperature (a surface temperature of the substrate) by means of vacuum evaporation technique. A resistivity is 6.2×10−6 ΩM in film formation.
The p-type GaAs layer 113 is formed at a carrier concentration of 1×1019 cm−3, and carbon tetrabromide (CBr4) is used as an additive raw material. Such CBr4 may be used for an additive raw material in another p-type layer. Furthermore, diethylzinc (DEZ) and dimethylzinc (DMZ) may also be used as other additive raw materials in a p-type layer, while silane (SiH4) may be used as an additive raw material for an n-type layer.
The p-type electrode 18 is formed in a matrix shape having 125 μm diameter by evaporating nickel in 20 nm, and gold in 1000 nm, respectively, in this order.
The n-type electrode 19 is formed on the whole surface of the backside of an LED (a surface on which no layer is formed in a semiconductor substrate) in such a manner that gold-germanium in 60 nm, nickel in 10 nm, and gold in 500 nm, respectively, in this order, and then, alloying for electrode is carried out at 400° C. for five minutes in nitrogen gas atmosphere.
However, according to the above-described LED, since adhesion of the p-type GaAs layer 113 of the uppermost layer to the transparent conductive film 17 is not sufficient, there is such a problem that exfoliation appears in the transparent conductive film, resulting in decrease of an yield. Besides, there is also such a disadvantage that sides of the transparent conductive film become irregular, so that a backward voltage becomes low. In the LED shown in FIG. 2, when a condition for measuring the backward voltage is 10 μA and a voltage at that time is −5 V or less, the result is considered to be poor. A light emission output of the LED is 2.50 mW, and a forward operative voltage is 1.98 V at the energization of 20 mA. However, it is confirmed that there is failure due to exfoliation of the ITO film 7 and failure in backward voltage with respect to 20% of the LEDs.
For instance, a manner for inserting an intermediate band gap layer between a GaAs layer and a cladding layer is known for moderating band discontinuity between the GaAs layer and the cladding layer. Even in this manner, however, although a forward voltage can be reduced at a certain degree, deterioration in exfoliation of the transparent conductive film and backward direction characteristics cannot be improved as a matter of course. This is because a layer which is in contact with the transparent conductive film is a GaAs layer. In addition, provision of such intermediate band gap layer between the GaAs layer and the cladding layer increases the cost therefor.
When CBr4 is used as a raw material for adding C, sufficient characteristics can be attained in a first time growth. However, when growth is repeated continually, a light emission output goes down extremely as low as about 50% in a second time growth and thereafter, so that there is a problem of poor reproducibility. For specifying a cause for the problem, the present inventors conducted SIMS analysis on epitaxial wafers grown after the second time growth and thereafter. As a result, it has been found that carbon (C) and oxygen (O) of high concentrations exist in the epitaxial wafers. Based on the fact, it is considered that since raw material CBr4 is used, the high-concentration C and O remain in a growing furnace in the first time growth, and the remained C and O are mixed into epitaxial wafers in the second time growth and thereafter, resulting in decrease in light emission output.